EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 



339 



proper remains relatively short. In the teeth the 

 incipient dolichocephaly appears in the following 

 characters: (1) Incisors somewhat compressed, oppo- 

 site pairs ranged in convergent series; (2) canines 

 laterally compressed, or lanceolate, rather than 

 rounded; (3) premolars and molars generally with 

 elevated crowns somewhat compressed transversely, 

 and with decidedly compressed crescents and sharply 

 pointed cones; (4) conules reduced or vestigial; (5) 

 first inferior premolars laterally compressed, with dia- 

 stemata on either side; (6) molars laterally compressed. 



In the skull we first observe the elongate, deep, and 

 narrow premaxillary symphysis and the corresponding 

 form of the "median suture." This is the generic 

 character originally pointed out by Marsh and em- 

 phasized by Earle, in contrast with the shallow, 

 rounded symphysis and median suture of Palaeosyops. 

 The anterior aspect (fig. 295) of the symphysis is very 

 characteristic of the species of this genus as compared 

 with Palaeosyops and Mesatirhinus but is not greatly 

 different from the Manteoceras type. The zygomata 

 bend outward widely but not so much as in Palaeosyops; 

 they are deeply extended vertically into flanges. The 

 external auditory meatus remains widely open below. 



Distinctions from Manteoceras. — The distinctions 

 from- Manteoceras are seen in a number of prominent 

 characters in the Telmatherium series: (1) The horn 

 rudiments are less prominent and the facial concavities 

 less pronounced (T. ultimum); (2) the malar section 

 below the orbits in T. cultridens is roundly angulate 

 and in T. ultimum it is more rounded, approaching 

 that in Palaeosyops, whereas in Manteoceras it is 

 sharply angulate externally, foreshadowing the shelf- 

 like flattening and rudiment of the infraorbital shelf 

 which is so prominent a feature in MesatirJiinus and 

 Dolicliorliinus; (3) the canines are elongate, laterally 

 compressed, and lanceolate, while in Manteoceras they 

 are suboval and incurved rather than vertical; (4) the 

 lateral superior incisors of TelmatJierium (Pis. LV^ 

 LXIV) rapidly increase in size, progressively becoming 

 caniniform (T. validum, T. ultimum), but in Manteo- 

 ceras the lateral incisors are moderately large and 

 increase in size progressively, though the disparity 

 between i^ and i^ is less marked than in T. cultridens: 

 in DolicTiorTiinus they progressively diminish in size, 

 but the lateral incisor, while the largest of the three, 

 is both relatively and absolutely smaller than in 

 Telmatherium and Manteoceras; (5) in Telmatherium 

 ultimum the ectolophs of the superior premolars (PI. 

 LXV) exhibit a very pronounced development of the 

 cingulum, which rises in a festoon upon the protocone, 

 producing an asymmetry of the outer face (a highly 

 progressive character), whereas in Manteoceras the 

 cingula are less pronounced and the protocones and 

 tritocones are less subequal on the ectoloph; (6) in 

 Telmatherium the deuterocones of the premolars are 

 longitudinally compressed (Pis. LXIII, LXIV, LXV, 



fig. 291), with a tendency to a ridged apex, which 

 becomes more marked in T. validum and very decided 

 in T. ultimum, whereas in Manteoceras the deuterocones 

 of the premolars are more oval or conical; (7) in Tel- 

 matherium the mesostyles of the superior molars are 

 sharply compressed (Pis. LXIV, LXV, fig. 292), the 

 buttress rising into a horizontal ridge, which becomes 

 a very decided character in T. validum and T. ultimum, 

 while in Manteoceras the mesostyles are more robust 

 and rounded; (8) in the members of both genera the 

 conules tend rapidly to disappear owing to the lateral 

 compression of the crown and the elongation of the 

 ectoloph, but the ectolophs in Telmatherium seem to 

 be even more elongate, progressive, and trenchant 

 than in Manteoceras. 



There are, however, some peculiar features which 

 distinguish the incipient dolichocephaly of this phyhun 

 from the more pronounced dolichocephaly seen in the 

 genera Mesatirhinus and Dolichorhinus — namely, the 

 free nasals are relatively short; the sagittal crest is 

 elongate and relatively persistent; the basicranial 

 region is relatively abbreviate. These differences are 

 consistent with the general law that dolichocephaly is 

 a process of differential growth of different parts of the 

 cranium, not all parts being elongated equally. 



Affinity to Manteoceras. — There are important fea- 

 tures in which T. cultridens, from the upper Bridger, 

 the earliest known member of this series, resembles the 

 contemporary representatives of Manteoceras, as shown 

 in a comparison of Figures 290 and 308. There appears 

 to be a similar development of the nasofrontal horn 

 rudiment and a somewhat similar concavity in front of 

 the orbits, though unfortunately this region of the 

 type of T. cultridens is fragmentary (fig. 290). A de- 

 cided resemblance to Manteoceras and Limnohyops and 

 distinction from Palaeosyops are seen in the form of the 

 nasal bones, which in T. ultimum are elongate but 

 with a short free portion which is laterally decurved 

 and truncate instead of pointed distally (contrast 

 Palaeosyops). 



With these exceptions the progressive affinities of 

 Telmatherium to Manteoceras and Mesatirhinus appear 

 to be adaptive and convergent characters rather than 

 ancestral or genetic characters. 



It thus appears that the distinctions from Manteo- 

 ceras outweigh the resemblances and that the resem- 

 blances to Manteoceras and Mesatirhinus are in part 

 attributable to parallel or convergent adaptation, in 

 part to similarity of origin. '° 



Progressive and conservative or stationary characters. — 

 It appears at present that the horn rudiments are not 

 progressive in the telmatheres; they are found to be 

 even less prominent in the Uinta T. ultimum than 

 in the Bridger T. cultridens — a feature possibly com- 



" W. K. Gregory regards the species T. cultridens as linked by intermediate stages 

 (Am. Mus. 12193, 12194) with M. manteoceras and as very closely related in all char- 

 acters, a resemblance not due to convergence. T. cultridens, according to this view, 

 is intermediate between the manteoceratine and the palaeosyopine divisions. 



